1. Field
The present art relates to a high-sensitive resistance measuring device and method of solder bumps which monitor the solder connection state of a package such as a ball grid array mounted on a circuit board by the solder bumps and, particularly, relates to the high-sensitive resistance measuring device and monitoring method of the solder bumps which detect and monitor minute variations of resistance caused by cracks generated in the solder bumps by stress.
2. Description of the Related Arts
Recently, in a ball grid array package (hereinafter, referred to as a “BGA package”) that mounts plural processors on a system board of, for example, a server, power source pins and signal pins are increased to 1000 pins or 2000 pins due to enhancement of functions and increase of power consumption of the server, and the size of the BGA package is increased along with that. When the size of the BGA package is increased in this manner, due to mutual difference in thermal expansion coefficients caused by thermal cycles of the system board, the BGA package, and semiconductor ICs constituting processors having different materials, stress is repeatedly applied to the solder connection units between solder bumps of the BOA package and the circuit board, and there is a danger that stress distortions of the solder bump connection units are increased, thereby causing damages such as minute cracks. The size of the circuit board such as a system board on which BGA packages are mounted has also been increased; and, when the size of the circuit board is increased, there is a danger that board deformation caused during handling or board assembly processes of the circuit board may cause damages such as minute cracks in solder bumps of the BGA package. When the minute cracks are generated in the solder bumps of the BGA package, the cracks gradually grow along with time elapse; and, when the development state of the cracks exceeds a certain limitation, the cracks rapidly grow, thereby rapidly increasing the resistance of the solder bumps and sometimes leading to rupture, and these damages such as the minute cracks are a cause of long-term reliability deterioration. The damages of the solder bumps in the BGA package cause increase of the resistance of the solder bump connection units; however, in order to detect initial damages of minute crack generation, extremely minute resistance variations have to be detected. Conventionally, as a method of detecting minute resistance variations, for example, a four-terminal method is known. In the four-terminal method, a circuit which supplies the current for resistance measurement and a circuit which detects the voltage generated in resistance are independent; therefore, two terminals for current supply and two terminals for voltage measurement, i.e., four terminals in total are provided. The voltmeter used in the four-terminal method has extremely high internal resistance, and almost no current flows through the circuit at the voltage measurement side; therefore, the voltage drop caused by, for example, contact resistance of a solder bump connection unit serving as a measurement object or wiring resistance of the measurement device can be ignored, and merely the resistance of the solder bump connection unit can be accurately measured.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. 2003-043091    [Patent document 2] Japanese Patent Application Laid-Open Publication No. H7-104023
However, detection of resistance variations of the solder bump connection unit using such a conventional four-terminal method has a problem that the detection sensitivity is insufficient, and minute resistance variations cannot be accurately detected. As the conventional four-terminal method, for example, when a voltmeter having minimum resolution power of 1 mill volt and a measurement range of 100 mill volts is presupposed, furthermore, initial resistance of the solder bump connection unit is presupposed to be 1 ohm, and the current value of the constant current that flows through the solder bump connection unit is presupposed to be 1 mill ampere, the initial electric potential is 1 mill volt, and the electric potential variation of 1 mill volt shows the resistance variation of 1 ohm. In order to detect the resistance variation corresponding to the development state of the cracks of the solder bump connection unit, the resolution power of about 0.01 percent of the initial resistance is needed. Thus, if the initial resistance is 1 ohm, a resistance variation of 0.01 ohm has to be detected. The resolution power of resistance detection can be enhanced in this manner by increasing the current value of the constant current that flows through the solder bump connection unit by the four-terminal method. Then, when the current value of the constant current is increased to 100 mill amperes, the voltage variation of 1 mill volt can represent the resistance variation of 0.01 ohm. However, since the initial resistance of the solder bump is 1 ohm, the initial value of the measured voltage is 100 mill volts, which is the max of the range; and, when the resistance thereafter is increased by 0.01 ohm, the measured voltage overflows to 101 mill volts, and minute resistance variations cannot be measured, which is a problem. Furthermore, when a particular solder bump connection unit is subjected to quality determination by detecting minute resistance variations, since the resistance is varied also by the temperature variations of the entire package, the resistance variation caused by damages such as cracks of the solder bump connection unit and the resistance variation caused by the temperature variation cannot be distinguished from each other, and the resistance variation caused by the damages of the solder bump connection unit cannot be detected, which is a problem.